Supported sulfur compositions, such as alumina which has been impregnated with liquid elemental sulfur, are effective trialkyl arsine sorbents. These materials and their use as trialkyl arsine sorbents have been described in U.S. Pat. Nos. 5,085,844 and 5,360,779. The present invention is directed to an improved method of making supported sulfur compositions which exhibit enhanced trialkyl arsine sorption capacity.
It is an object of this invention to provide an improved process for preparing supported sulfur compositions which are effective as trialkyl arsine sorbents. It is a further object of this invention to provide supported sulfur compositions made by the process of this invention and a method for absorbing trialkyl arsine using the supported sulfur compositions of this invention. Particular objects and advantages of this invention will become apparent from the detailed description and the appended claims.
In a process for preparing a supported elemental sulfur composition, the improvement comprises contacting molten elemental sulfur and an inorganic support material in an inert atmosphere thereby preventing the formation of sulfur oxides which are ineffective as absorbents for trialkyl arsines.
Preferably, the inorganic support material is alumina. It is presently preferred to employ a solid combination consisting essentially of elemental sulfur and inorganic support material that has been prepared by impregnating the inorganic support material (preferably alumina) with molten elemental sulfur at a temperature above the melting point of elemental sulfur in an inert atmosphere, and then lowering the temperature below the melting point of elemental sulfur while retaining the composition in an inert atmosphere so as to afford solidification of the elemental sulfur on the inorganic support material.
Any suitable, effective inorganic support material can be employed as the support (carrier) component of the solid combination of elemental sulfur and an inorganic support material which is the final product of this invention. Preferably, the support material is selected from the group consisting of alumina, fluorided alumina (i.e., alumina which has been treated with HF or NH4HF2 under conditions as to incorporate fluoride ions into the crystal lattice of alumina), aluminum phosphate, magnesia (MgO), silica, titania (TiO2), zirconia (ZrO2), hafnia (HfO2), zinc oxide, zinc aluminate (ZnAl2O4), aluminates of alkaline earth metals (i.e., of Be, Mg, Ca, Sr, Ba), zinc titanate (Zn2TiO4), titanates of alkaline earth metals, activated carbon, and mixtures of two or more than two of the above materials. Presently more preferred support materials are alumina, silica, titania, activated carbon, zeolites and mixtures of two or more of these materials. Particularly preferred is alumina.
The elemental sulfur component is combined with the inorganic support material by impregnating the support material with molten sulfur, followed by cooling below the melting temperature of sulfur with the processes of impregnating and cooling below the melting temperature of sulfur carried out in an inert atmosphere. Generally the elemental sulfur will be contacted with the inorganic support and the temperature then raised above the melting point of elemental sulfur to impregnate the inorganic support. The temperature is then decreased so that the sulfur solidifies on the inorganic support while the inert atmosphere is maintained thereby providing solid sulfur on an inorganic support.
Generally, the sulfur content in the supported composition is in the range of from about 1 to about 50, preferably from about 3 to about 25, weight-% elemental S. It is within the scope of this invention to have, in addition to elemental sulfur, metal oxides and/or metal-sulfur compounds (such as Fe(III) oxide and/or sulfite and/or sulfate or the corresponding compounds of Co and/or Ni and/or Mn) present in the supported composition.
The supported elemental sulfur composition produced by this invention can have any suitable surface area (preferably about 10-1000 m2/g, as measured by the B.E.T. method employing N2), any suitable shape (such as spherical, cylindrical, ring-shaped, trilobal, etc.), and any suitable particle size (such as about 0.2-20 mm diameter of spherical particles).
This invention yields improved results in the removal of trialkyl arsines from fluid streams that contain trialkyl arsines as compared to using supported sulfur compositions made in every way by the same process except for carrying out the impregnation of the support with molten sulfur under an inert atmosphere.
The term xe2x80x9ctrialkyl arsinexe2x80x9d, as used herein, refers to compounds having the general formula of R3As, wherein each R is a radical independently selected from among alkyl groups (straight or branched), preferably having a 1-6, more preferably 1-3, carbon atoms. Particularly preferred trialkyl arsines are trimethyl arsine, triethyl arsine, dimethyl ethyl arsine and diethyl methyl arsine.
Any suitable liquid or gaseous fluid stream which contains trialkyl arsine can be used as feed in the process of this invention. Preferably, the feed is gaseous. Non-limiting examples of suitable feeds are: natural gas, gaseous petroleum fractions comprising paraffins and olefins containing 1-6 carbon atoms per molecule and gaseous products from thermal and catalytic cracking of petroleum, shale oil or coal. Generally the gases comprise methane, ethane, ethylene, propane, propylene, n-butane, isobutane, butenes and the like. These gas streams can contain other impurities, such as hydrogen sulfide, carbonyl sulfide (COS), mercaptans, organic sulfides, carbon monoxide, carbon dioxide, inert gases (N2, He, Ne, Ar),and the like. The process of this invention is effective even when H2S is present.
Other arsenic compounds may also be present in the fluid stream which is treated by the process of this invention, such as AsH3, RAsH2, R2AsH, R3AsO (trialkyl arsine oxides), R3AsS (trialkyl arsine sulfides) and the like; wherein R is an alkyl group, as defined above. It is also possible to have triphenyl arsine, dialkyl phenyl arsines, dialkyl cycloalkyl arsines and the like present in the feed. Preferably free oxygen is substantially absent from the feed.
Generally, the total concentration of the trialkyl arsines in the preferably gaseous feed is in the range of from about 1 ppb (1 part by weight of trialkyl arsenic per billion parts by weight of feed) to about 0.1 percent arsenic. The concentration of other impurities and the exact composition of the feedstock will vary widely from feedstock to feedstock.
Any suitable contacting conditions can be employed in the sorption process of this invention. Generally, the temperature in the contacting zone is in the range of from about xe2x88x9220 to about 50xc2x0 C. Generally, the pressure in the contacting zone is in the range from about 1 to about 500 atm., preferably about 1 to about 70 atm. Generally, the hourly space velocity of the gaseous feed in the contacting zone is in the range from about 10 to about 20,000volume of feed/volume of sorbent/hour, preferably from about 1,000 to about 10,000volume/volume/hour, measured at about 25xc2x0 C.,/1 atm. Generally, the contacting is continued until trialkyl arsine breakthrough occurs, i.e., when the treated product contains more trialkyl arsines than can be tolerated, such as an amount of about 50 ppb.
Treatment of the feed stream in accordance with the process of this invention can be carried out in any suitable manner. In one embodiment of the invention, a bed of the sorbent is placed in a fixed bed in a confined zone and a fluid stream, preferably a gas, is passed therethrough in either upward or downward flow. Other less preferred methods of treatment include a fluidized operation in which the feed and the sorbent particles are maintained in a state of turbulence under hindered settling conditions in a confined zone, moving bed operations in which the sorbent passes as moving bed countercurrently to or cocurrently with the feed. In a fixed bed operation of a continuous process the flow of fluid can be rotated between two or more sorbent beds with at least on bed in sorbent operation with another in regeneration mode. Continuous processes are preferred, but it is understood that batch operation is also possible.
The following examples are provided to further illustrate this invention. The examples are not to be construed as unduly limiting the scope of this invention.